LNG (liquefied natural gas) has become a particularly important energy source in a number of countries such as Japan, Korea, Taiwan, and various countries of Europe which are dependent upon outside energy sources, and many areas of the world depend on LNG as their primary source for natural gas. Natural gas is routinely liquefied in Saudia Arabia and Indonesia (by lowering its temperature to about -260.degree. F.), thus increasing its density about 600 times. It is then shipped in special insulated tankers to Europe and the Far East, particularly Japan, where it is stored in insulated tanks until required. When gas is required, the LNG pressure is increased by pumps until it matches the pipeline pressure and then it is vaporized. This step requires a large addition of heat to the LNG before it can be added to the natural gas distribution pipeline network on an "as needed" basis. Such pipeline networks can be operated at quite varied pressures. For natural gas that is to be utilized in the immediate vicinity, a pressure of less than 50 psig is frequently used. For more distant supply areas, pressures of about 250 psig are frequently utilized. In some cases, longer distance high pressure distribution lines may utilize pressures of 500 psig and even higher.
Since LNG terminals at the receiving points are nearly always located near water to accommodate ocean-going tankers, sea water is usually available to provide the necessary heat of vaporization. It has long been recognized that the refrigeration potential of such vast quantities of LNG is considerable, and it has been a real challenge to attempt to economically use the cold energy that is available. Recently however, the refrigeration potential of LNG has received increasing attention. This situation is described by J. Maertens in his article entitled, "A Design of Rankine Cycles for Power Generation from Evaporating LNG" which appeared in Rev. Int. Froid. 1986, Vol. 9, pp. 137-143. Maertens indicated that, in addition to the generation of electrical energy, there have been efforts made to use the LNG cold potential, in Japan, to produce solid CO.sub.2 (dry ice) at -110.degree. F., to cool entering air for an air separation plant which may operate at about -320.degree. F., or to refrigerate cold storage food warehouses at about -20.degree. F.
The generation of electrical power has been one of the more frequently investigated uses of the cold energy potential of LNG. U.S. Pat. No. 2,975,607 shows the recovery of power during the vaporization of LNG by a single expansion of a condensable circulating refrigerant, such as propane or ethane, and suggests the use of sea water to provide an ambient heat source. The use of a cascade refrigeration system employing ethane and then propane for vaporizing LNG streams and recovering power by the use of expanders is shown in U.S. Pat. No. 3,068,659. U.S. Pat. No. 3,183,666 uses a gas turbine which burns methane to vaporize the working fluid, i.e. ethane, before it is expanded and then condensed against the vaporizing LNG. More recent U.S. Pat. No. 4,330,998 discusses the potential problems that can occur from the use of sea water in a confined area from the standpoint of "cold water pollution". This patent proposes to use a circulating freon stream which can be expanded to drive a turbine, to create mechanical energy and ultimately generate electricity. This patent specifically discloses the use of LNG to condense nitrogen, which is subsequently expanded to create power after being pumped to high pressure and vaporized by condensing freon which is used as the working fluid in a main power plant. U.S. Pat. No. 4,437,312 discloses the vaporization of LNG through a series of heat exchangers in which it absorbs heat from two different multicomponent streams of gases, with one stream containing four hydrocarbons and some nitrogen while the other stream contains a three hydrocarbon mixture. Both streams are expanded in turbines to create electrical power. The Maertens paper also discusses various power cycles for using the LNG in electrical power generation.
All of the previously directed uses of LNG refrigeration have certain drawbacks. These refrigeration use cycles often experience the following disadvantages: the inefficient use of the low temperature potential (e.g., using -240.degree. F. LNG which vaporizes at 50 psig to cool CO.sub.2 to dry ice temperatures of -110.degree. F.); the quantities of heat don't match, i.e., the small quantity of air separation products produced and sold in liquefied form compared to the much larger amount of LNG which must be vaporized; the liquefication temperatures don't specifically match, causing the use of temperature-lowering devices; and/or the use cycle of natural gas from a time standpoint doesn't match the use cycle of the partner process.
The electric power generating cycles discussed by Maertens attempt to rectify such drawbacks by using the refrigeration potential of the LNG in combination with certain complex intermediate working fluid cycles. However, the Maertens cycles are both complex and expensive. They must be sized to handle varying LNG flows, which makes them either expensively over-sized for much of the time or, if undersized for the peaks, wasteful of much of the refrigeration.
All of the aforementioned power cycles suffer from another defect: namely, they make electricity only when natural gas is being used. Therefore, they are not weighted towards the "peak hours" of electrical demand, when electricity has a much higher value.
Electric utility companies, whatever their source of energy, have recently endeavored to make better use of their base load power plants and have considered storing electrical power. They have also investigated the employment of highly efficient power generation systems to meet peak load demands. One highly efficient way of electrical power generation is to employ a gas or oil-fired combustion turbine as a part of a combined-cycle system. In such a system, the heat rejected by the higher temperature or topping cycle is used to drive the lower temperature cycle to produce additional power and operate at a higher overall efficiency than either cycle could achieve by itself. The lower temperature cycle is referred to as the "bottoming cycle", and typically most bottoming cycles have been steam-based Rankine cycles, which operate on the heat rejected, for example by a combustion turbine exhaust. This peak consideration led Crawford et al., in U.S Pat. No. 4,765,143, to propose a power plant using a main turbine to drive a generator with the use of carbon dioxide as the working fluid in a bottoming cycle. This system has the ability to generate a large amount of electrical power during periods of peak usage throughout the week while storing excess power that is available during non-peak hours. This patent also suggests the possible use of LNG to provide the refrigeration to the CO.sub.2 power cycle.
A paper entitled "SECO.sub.2 (Stored Energy in CO.sub.2) Retrofit CO.sub.2 Bottoming Cycles with Off-Peak Energy Storage for Existing Combustion Turbines," by J. S. Andrepont et al. studied the cost and performance of combined cycle gas turbines with such a CO.sub.2 power cycle for peaking service under various conditions; the required mechanical refrigeration equipment was very expensive to install and operate. While the LNG-SECO.sub.2 combination suggested in the above patent broadly contemplated another potential use of LNG's refrigeration, it made no attempt to efficiently take advantage of LNG's very low temperature potential, because the CO.sub.2 triple point occurs near -70.degree. F. and only a limited temperature difference is required for heat transfer. While the varying LNG vaporization demand might indicate that high temperature differences across the heat exchanger be employed to minimize equipment cost, the use of a 30.degree. F. temperature approach requires a low temperature of only -100.degree. F. Therefore, the ample available refrigeration of LNG below -100.degree. F. would not be well utilized with a direct heat exchanger configuration.
Few of the existing systems designed to utilize the available LNG refrigeration appear to have true commercial potential. Low temperature uses of LNG are often at inconvenient levels or not well matched to utilize the cold potential without any limitation upon LNG's primary role, which is to supply natural gas to a distribution network at a variety of pressures and appropriate temperatures. Therefore, although these various systems may have certain advantages in particular situations, the electrical power-generating industry and the natural gas pipeline industry have continued to search for more efficient and economical systems.